1. Field of the Invention
This invention relates to a thermally assisted magnetic head capable of high-density recording, and to a manufacturing method for such a head.
2. Related Background Art
The rising recording densities of hard disk devices have led to demands for further improved performance of thin film magnetic heads. As thin film magnetic heads, composite thin film magnetic heads, having a structure in which a magnetoresistance (MR) effect element or other magnetic detection element is laminated with an electromagnetic coil element or other magnetic recording element, are widely used; by means of these elements, data signals can be read from and written to the magnetic disks that are the magnetic recording media.
In general, magnetic recording media are discontinuous media resulting from aggregation of so-called fine magnetic particles, which each of the fine magnetic particles having a single-domain structure. Here, one recording bit comprises a plurality of fine magnetic particles. Hence in order to raise the recording density, the fine magnetic particles must be made smaller, and the irregularities at the boundaries between recording bits must be made smaller. However, if the fine magnetic particles are made too small, a decline in thermal stability of magnetization, accompanying the smaller particle volume, becomes a problem.
An index of the thermal stability of magnetization is given by the quantity KUV/kBT. Here KU is the magnetic anisotropy energy of the fine magnetic particles, V is the volume of one fine magnetic particle, kB is the Boltzmann constant, and T is the absolute temperature. Making the fine magnetic particles smaller mean precisely that V is made small, and without further changes, as KUV/kBT grows smaller, thermal stability is lost. In order to address this problem, simultaneously making KU larger is conceivable; however, this increase in KU would cause an increase in the coercivity of the recording media. On the other hand, the write magnetic field intensity of the magnetic head is substantially determined by the saturation magnetic flux density of the soft magnetic material comprised by the magnetic pole within the head. Hence if the coercivity exceeds a tolerance value determined by the limit of this write magnetic field intensity, writing is no longer possible.
As methods to resolve such problems with the thermal stability of magnetization, so-called thermally assisted magnetic recording methods have been proposed, in which, while using magnetic material with a large KU, heat is applied to the recording media immediately before applying the write magnetic field, in order to lower the coercivity and perform writing. Such methods can be broadly divided into magnetic-dominant recording methods and light-dominant recording methods. In magnetic-dominant recording methods, the main component of writing is an electromagnetic coil element, and the light irradiation diameter is large compared with the track width (recording width). On the other hand, in light-dominant recording methods, the main component of writing is a light irradiation portion, and the light irradiation diameter is substantially the same as the track width (recording width). That is, whereas in magnetic-dominant recording methods the spatial resolution is determined by the magnetic field, in light-dominant recording methods the spatial resolution is determined by the light.
Japanese Patent Laid-open No. H10-162444 discloses technology to record ultra-fine magnetic domain signals onto a magneto-optical disc with an ultra-fine light beam spot, utilizing a magnetic head employing a solid emulsion lens. Also in S. Miyanishi et al, “Near-field assisted magnetic recording”, IEEE Transactions on Magnetics, 2005, Vol. 41, pp. 2817-2821, technology is disclosed in which a U-shape near-field probe formed on a quartz slider is employed to generate evanescent light and a magnetic field, to form a recording pattern of approximately 70 nm.
Japanese Patent Laid-open No. 2001-255254, Japanese Patent Laid-open No. 2003-114184, and Japanese Patent Laid-open No. 2006-185548 disclose a thermally assisted magnetic head in which a conductive plate-type near-field light generation portion is positioned on a medium-opposing surface, and by irradiating with light from the side opposite the medium side, near-field light is generated. A pointed tip portion is formed at one end of the near-field light generation portion, and the near-field light is mainly irradiated from this tip portion.
Japanese Patent Laid-open No. 2004-158067 discloses technology in which a scattering body comprising a near-field probe is formed in contact with the main magnetic pole of a perpendicular magnetic recording single-pole write head, so as to be perpendicular to the recording medium. In this technology of the prior art, the near-field light generating element and the main pole of the perpendicular magnetic recording single-pole magnetic write head are placed at a distance in the linear recording direction. At the same time, a structure is employed in which the main pole is positioned on the substrate side from the near-field light generating element (the light irradiation portion is positioned on the trailing side of the magnetic pole). In this case, when the disk rotation direction in hard disk drives of the prior art is adopted, after heating the medium, the magnetic field is applied after the heating portion has passed through approximately one rotation of the disc. This is a method of use in which in which the cooling efficiency of the medium must be made considerably poor in order to enable use; if the write speed is considered, this structure is not realistic. Moreover, for the construction of Japanese Patent Laid-open No. 2004-158067, the magnetic field which can be applied is not adequate, and in particular, application of this head is difficult for recording media having two-stage coercivity-temperature characteristics such as described in Jan-Ulrich Thiele et al, “Magnetic and structural properties of FePt/FeRh exchange spring films for thermally assisted magnetic recording media”, IEEE Transactions on Magnetics, 2004, Vol. 40, No. 4, pp. 2537-2542).
In Japanese Patent Laid-open No. 2005-4901, a light irradiation portion is provided in proximity to the trailing-side end of the recording magnetic pole. Using this technology, by improving the recording magnetic field gradient and similar, a magnetic field can be applied to the heating portion, but the design margin is not necessarily large, and practical realization is not easy.
When a near-field light-generating element is positioned on the substrate side from the main pole, it is preferable that the recording magnetic field be imparted from the magnetic pole after the medium is heated; however, if there is too great a distance between the two elements, the effect of heating is lost. Also, in order to adopt a construction in which the near-field light-generating element is positioned on the substrate side of the main pole, a path for the light to propagate, that is, an optical waveguide, must also be provided on the side below the perpendicular magnetic recording single-pole write head (the substrate side). Such a construction is for example described in Japanese Patent Laid-open No. 2005-190655.
In Japanese Patent Laid-open No. 2006-185548, technology is disclosed in which an optical waveguide is embedded within the main pole of a perpendicular magnetic recording single-pole write head; however, it is extremely difficult to form such an optical element in a main pole of width 20 nm or less to attain a recording density of 1 Tbits/in2, and from the standpoint of efficiency of light use as well, such a design is not desirable.
As explained above, although various technologies are known, a thermally assisted magnetic head capable of performing practical high-density recording has not been obtained. When considering a construction in which a near-field light-generating portion (plasmon probe) is irradiated with laser light, it is thought to be preferable to provide the near-field light-generating portion in the tip face of the core of the optical waveguide through which the laser light propagates. The optical waveguide is formed by enveloping a region of high refractivity (core) within a region of low refractivity (cladding). In order to ensure functioning as an optical waveguide, the thicknesses of the two regions must be designed to be approximately equal to or greater than the wavelength of the light used.
Hence when using a blue laser as the light source in high-density optical recording, for example, a core and cladding thickness of approximately 400 nm or greater each are required; in this case, the near-field light-generating portion and the main pole are greatly separated, and there is the problem that a magnetic field cannot effectively be applied to the heated medium region.